The present invention relates to an improvement in an electric current supply or feeding apparatus, and more particularly to an apparatus having a heating unit for dissipating condensed moisture or fog from a window of a vehicle.
A known heating unit for use on a vehicle includes a heating element, for example heating wire, arranged in a rear glass window for dissipating fog therefrom by heat generation.
FIG. 1 shows one example of a conventional apparatus which has been used for feeding electric current to a defogger. In FIG. 1, there are shown a power source 1, an ignition switch 2, a fuse F1 connected to the ignition switch 2 in series and a fuse F2 connected to the power source 1 in series, the fuses F2 and F1 being connected to an input terminal 6 of a contact 4 and an exciting coil 18 respectively in an electromagnetic relay 3. The electromagnetic relay 3 has its output terminal 8 connected to a defogger 10 and an indicator lamp 11. A defogger starting switch 12 for actuating the electromagnetic relay 3 has its ON contact 13 connected to the fuse F1 and its OFF contact 14 connected to ground. A manipulation knob 15 of the starting switch 12 is designed to automatically return back to its neutral position after connection with the ON contact 13 or the OFF contact 14. An electronic timer 16 is adapted to respond to a signal from the starting switch 12 to actuate the electromagnetic relay 3 for a preset duration. A power input terminal 17 of the electronic timer 16 is connected with the fuse F1. The exciting coil 18 of the electromagnetic relay 3 has its one end connected with the fuse F1 and its other end with the electronic timer 16 respectively.
To actuate the prior art apparatus, the driver turns on the ignition switch 2 thus enabling operation, and then operates the manipulation knob 15 of the starting switch 12 provided on a front of instrument panel (not shown) of the vehicle, thus causing manipulation knob 15 to come into contact with the ON contact 13. As a result, a timer actuation signal is generated by the starting switch 12 and sent to the electronic timer 16 for actuating the timer 16, and while the timer 16 is operating, electric current flows to the energizing coil of the electromagnetic relay 3. The electromagnetic relay 3 closes contact 4 during this situation. In consequence, electric current flows from the power source 1 to the electromagnetic relay 3 via the fuse F2, and is further sent therefrom to the heating unit 10 and to the indicator lamp 11. Upon the end of the preset duration, or when the timer 16 stops, the flow of electric current to the exciting coil 18 is interrupted with a simultaneous turning off the electromagnetic relay 3. The length of time for which the timer 16 continues to operate is preset to be sufficient for the heating unit 10 to heat and dissipate fog formed on the rear glass window, and normally it is preferable to preset the duration, for example, to 10 to 15 minutes. If fog has disappeared intermediately during operation of the apparatus, the driver may operate the manipulation knob 15 of the starting switch 12 to come into contact with the OFF contact 14, thereby causing the timer 16 to cease its operation for interrupting the flow of electric current to the heating unit 10.
There is a need for a larger electric current and increased current capability of the electromagnetic relay and the fuses when the defogger is being used in a freezing region and the like, i.e., a geographical region in which freezing climatic conditions may exist. To cope with such a need, a circuit arrangement shown in FIG. 2 may be used, if it is desired to reduce production costs by using electromagnetic relays and the fuses such as are generally otherwise employed in vehicles. In this arrangement the electromagnetic relay 19 having two contactors 4 and 5 is used for feeding electric current to the heating unit 10, while two fuses F2 and F3 are arranged in parallel for achieving increased fuse capacity in a manner that two circuits are arranged in parallel. In this instance, if either fuse F2 or F3 is blown, or disconnection is caused in either circuit, a remaining fuse will be also blown, because it will be subjected to the flow of all of the electric current formerly divided between the contacts 4 and 5 of the electromagnetic relay 19. Namely, at the moment of connecting the starting switch 12 with the ON contact 13, a large inflow of electric current takes place to another circuit sufficiently for blowing the fuse connected thereto. As a result, the heating unit 10 remains cold and the indicator lamp 11 also remains unlighted, thereby informing the driver of the breakdown of the electric current feeding apparatus.
However, the circuit arrangement mentioned above presents a problem in that heating unit 10, which is mainly made of silver and so exhibits the inherent temperature characteristics of silver in general, is liable to vary its electric resistance, depending on ambient temperature. That is the, electrical resistance of the heating unit 10 becomes high if ambient temperature becomes high, thereby preventing a large electric current from passing abruptly through the fuses F2 and F3, and the contacts 4 and 5 of the electromagnetic relay 19. Consequently, in a case where electrical resistance of the heating unit 10 increases its electric resistance in excess of a preset level, there occurs a phenomenon that a single remaining fuse will not be blown, even if all electric currents to the heating unit 10 flow into one circuit exclusively. A resultant larger inflow of electric current to the electromagnetic relay 19 in excess of an allowable limit for the relay contacts connected to the remaining fuse causes the defect that the contacts may be damaged or held in their ON condition by melting.